This invention is concerned with a method of repairing leaks in oil well casing which have developed in the outermost or production casing of an oil well installation, the leaks being caused generally by external corrosion although the method takes care of leaks formed by internal corrosion as well. Additionally, during the drilling of an oil well the outermost casing may have been damaged during the drilling operations and such damage may be remedied by use of the present method.
In one Texas oil field, many wells have their production casing string or the outermost string of casing damaged due to external corrosion at depths within the well ranging from 100 to 500 feet. The leaks in the production casing string require repairs since beam pumping units are employed to recover oil from fresh-water sensitive oil sands. Unless the leaks are repaired, the casing leaks result in stuck or worn pumps due to sand inflow, as well as damage to the permeability of the oil sand near the well bore due to inflow of fresh-water. This later damage is especially prevalent in sands that contain swelling clays.
Possible remedies for the casing leaks include installation of a cup packer, squeeze cementing, replacement of the upper production string, and installation of a protective string. The first three remedies are well known to the art and the last solution forms the subject matter of this invention.
The use of a cup packer is the simplest and cheapest of the above indicated solutions. An experienced maintenance crew can pull pipe from a well, install a cup packer, and replace production equipment within 12 hours. Though simple to accomplish and relatively inexpensive, the cup packer has several serious drawbacks. Installing the cup packer prevents fluid level surveillance by an echometer. Additionally, if gas pressure builds up sufficiently below the lower cup, the packer can leak as the lower cup momentarily collapses to equalize pressure. Further, with the upper and lower cups holding, gas pressure is held against the formation, limiting inflow of oil into the well. For these reasons, a cup packer is used only on an interim fix basis.
Squeeze cementing was carred out at numerous times on several of the leases within the Texas oil field with marginal success. Operations were carried out wherein cement was mixed with various weights of thixotropic cement additives in an effort to find the best cement recipe. Thixotropic cement made from class A cement weighted at 14.5 lbs. per gallon with 12.5 lbs. per sack of gilsonite gave the best results. Still, even a perfect squeeze job in badly corroded casing is only a temporary fix. In four different wells in the field, 10 cement squeezes were carried out successfully. At two other wells, 7 cement squeezes were unsuccessful. In an additional two wells, one cement squeeze was successful and the other was unsuccessful. Thus, of the 19 cement squeeze jobs carried out, 8 were unsuccessful.
At times, leaks in the upper part of a production casing string can be repaired by backing off and replacing the upper corroded sections of the production casing string. This method cannot be used in many oil fields because of certain overriding factors. For one, the field may have unconsolidated water sands at shallow depths. If there is no protective casing, these unconsolidated sands often collapse during the operation of pulling the production of outermost casing from a well. Additionally, many wells have their production casing strings cemented from the bottom all the way to the surface. Pulling cemented casing of this type would require a milling operation to free the pipe where it was cemented to the earth formation. After the cement was milled out, some of the unconsolidated sands might drop into the well.